The present invention relates to a photomask for use in optical lithography for the fabrication of semiconductor devices and so on, particularly to a photomask having phase shifters for shifting a phase of transmitted light, and to a method of fabricating the photomask.
Accompanying demands for larger-scales and higher integration of semiconductor integrated circuits in recent years, micronized patterning is required. To improve resolutions for optical lithography using photomasks there have been proposed phase shift masks using phase shifters for shifting phases of transmitted light. A phase shift mask of e.g., a line and space pattern of alternated light shielding and light transmitting areas is intended to improve resolutions of patterns by setting at. 180.degree. a phase difference between light transmitting areas with light shielding areas therebetween.
But in the case of a phase shift mask having a part in a light transmitting area, where a phase inversion of transmitted light takes place (i.e., an area (0.degree. area) with no phase shifter formed thereon and an area (180.degree. area) with a 180.degree. phase shifter formed on are adjacent to each other) a light intensity of the transmitted light is lowered at the part although the part is the light transmitting area. Consequently by using such phase shift mask, resists adversely remain at parts where patterns are not to be formed. Attention has to be paid to arrangements of phase shifters so as not to form parts in light transmitting areas, where no phase inversions take place.
To remove such limitation to the arrangement of phase shifters, a 90.degree. shifter, for example, is arranged at a part where a phase is inverted, whereby a sharp shift of a phase in a light transmitting area has been prevented.
This type of conventional photomask is shown in FIG. 12. FIG. 12A is a plan view of the photomask, FIG. 12B is a sectional view along the line A1-A1' of FIG. 12A, and FIG. 12C is a sectional view along the line B1-B' of FIG. 12A.
Chrome layers 102 are formed in lines on a mask substrate 100. To shift phase by 180.degree. in every two ones of light transmitting areas sandwiched by the chrome layers 102, phase shifters 104, 106 each for shifting a phase by 90.degree. are superposed on each other, and the lower phase shifter 104 alone is formed on borders of a light transmitting areas with none of the phase shifters formed thereon. In the borders of the light transmitting areas without the phase shifters a phase is bluntly shifted by 90.degree. , so that no sharp phase shifts take place.
Another conventional phase shift mask having phase shifters so arranged that the light transmitting area has no part where a phase is inverted in FIG. 13 (H. Ohtsuka et al., "Conjugate Twin-Shifter for the New Phase Shift Method to High Resolution Lithography", SPIE, Vol. 1463, pp. 112-123, Optical/Laser Microlithography IV (1991)). FIG. 13A is a plan view of the photomask, and FIG. 13B is a sectional view along the line A2--A2 of FIG. 13A.
In this conventional phase shifter, chrome layers 102 are formed on a mask substrate 100, and 90.degree. phase sifters 108 for shifting a phase by 90.degree. and 270.degree. phase shifters 110 for shifting a phase by 270.degree. are alternately formed on each of light transmitting areas sandwiched by the chrome layers 102.
A phase is inverted in the boundary parts between the 90.degree. phase shifter 108 and the 270.degree. phase shifter 110. But in FIG. 13, because of the chrome layers 102 on the boundary parts, the light transmitting area has no parts where a phase inversion takes place.
In the conventional phase shift mask shown in FIG. 12, in which a phase is shifted by 180.degree. in the light transmitting areas, the shifters 104, 106 each for shifting a phase by 90.degree. are superposed on each other. If a defect is found in a phase shifter of the photomask, it will be difficult to correct the shifter alone separately.
The conventional shift mask of FIG. 13 has the problem that a light intensity is so lowered in the boundary parts of the phase shifters 108, 110 that a focus margin upon an exposure is lowered.